Radio Access Technology (RAT) indicates the type of radio technology to access the Core Network (CN). Examples of RAT are the UTRA (UMTS Terrestrial Radio Access), e-UTRA (evolved UTRA also called Long Term Evolution), CDMA2000®, DECT (Digital Enhanced Cordless Technology), GERAN (GSM EDGE Radio Access Network), etc.
The different radio access technologies (RATs) are designed to fulfil specific requirements and characteristics in terms of throughput, data rate, range, mobility etc. Nowadays, third generation (3G) mobile communication systems based on a WCDMA (Wideband Code Division Multiple Access) RAT is being deployed all over the world. Furthermore, new RATs are being considered for evolved-3G (LTE) and other so-called fourth generation (4G) wireless telecommunication systems. The amount of RATs able to work in different frequency bands is increasing and being standardized.
One key capability of RATs such as 3G and LTE is that they can provide truly mobile broadband, i.e. the combination of high capacity, full mobility and coverage for the provision of data services.
New RATs or the evolution of those RATs are mainly focused on increasing the capability of the users to receive more and more data (increasing mobile broadband capabilities).
There are multi-RAT capable User Equipments (UEs) which perform searches (at switch on or following recovery from lack of coverage), for a registered or equivalent Public Land Mobile Network (PLMN) using all RATs that the UE supports, one RAT after another. All frequencies in all bands belonging to each RAT are searched, signal strength measured in order to connect the UE to a suitable cell(s) in a suitable PLMN(s) Cell/PLMN selection is usually based on the signal strength, i.e., signal to interference ratio (SIR) or signal to noise ratio (SNR), of candidate cells.
However, the achievable user throughput is limited by the amount of spectrum allocated to one specific RAT technology as well as by the current traffic experienced over that technology on a determined moment in time. This is due to the fact that the current system architectures keep focusing in establishing a connection for a single UE over a single RAT at the time.
In spite of spare capacity can be available on a RAT technology (spectrum) different from the single one currently used, there is currently no means to aggregate data from different RATs and their radio spectra cannot be used as a boost for data connection. Since just only one single RAT at the time can be used, the existing approaches to reach higher throughput are either providing higher amount of spectrum over the used RAT (2G, 3G or LTE), either the load on a specific carrier has to be decreased.